U.S. cities are seeing more 90-degree days than ever. Is yours one of them?
Since the 1970s, the number of extremely hot days recorded in a given year has increased by a month or more in some places.
Are U.S. cities seeing more ultra-hot days than they did decades ago? A new analysis indicates that’s the case for much of the country.
According to numbers calculated exclusively for National Geographic by the research group Climate Central, more than 30 cities—from Miami to Houston—now experience a month or more of additional days over 90°F than they did in 1970, before climate change caused significant warming. Some 30 others are reeling under 20 or more additional 90-degree days.
The increase in these extremely hot days poses problems for health, infrastructure, crops, and wildlife, among other concerns, climate scientists say.
Ninety-degree heat has become so common in summer that, in many places, it’s actually the cooler weather, not the heat, that draws the media’s attention, says Zachary Labe, a climate scientist at Climate Central. “This shows we’re subconsciously getting used to the record-breaking heat,” he says, as it becomes the norm.
Miami, which leads the list, now sees 83 additional days of 90-degree heat compared with 1970—that’s nearly three extra months. And McAllen, Texas, experiences 62 more—a whopping 171 days of the year at these temperatures on average—while Tucson, Arizona, has 41 more 90-plus days than before.
With so many added days, extreme heat is no longer confined to summer. In October of 2024, for example, the thermometer in Tucson reached 100°F. Kids were already well into their fall sports leagues at the time, so Fatima Luna, the city’s chief resilience officer, pulled her then eight-year-old son out of his team’s soccer game so he wouldn’t experience health problems triggered by the heat. “It’s a real danger. We have to adjust our lives to the increased heat,” Luna says.
When the average goes up, so do the extremes
Even cities with a reputation for cold are seeing significantly more days over 90 than in 1970. Helena, Montana, has 24 additional 90-degree days; Cheyene, Wyoming, has 17 more; Denver added 27.
This rise results from a world that is increasingly heating due to the heat-trapping effects of burning fossil fuel, experts say. Average temperatures on Earth in 2023 and 2024 were the hottest ever recorded; last year was the fourth warmest.
“When you warm up the average temperature of a region and/or the planet, you will also get more extreme-heat days,” says David Neelin, a professor of atmospheric and oceanic sciences at the University of California, Los Angeles. Neelin was not involved in Climate Central’s analysis but has published papers modeling past and future climate trends.
Each fraction of a degree of warming also increases the length of heat waves, Neelin’s study found, with the hottest heat waves becoming longer than ever. In many cases, more back-to-back extremely hot days happen because climate change has increased the chance that a ridge of high pressure, colloquially known as a heat dome, excessively lingers over an area before wind currents can steer it away.
Health, agriculture, and infrastructure concerns
This increase in extreme heat is harmful to people because bodies may have trouble cooling when the temperature spikes. Days when the heat index—a measure of temperature combined with humidity—reaches 90 degrees spark the National Weather Service to issue extreme caution alerts.
Such temperatures can cause heat cramps and heat exhaustion, which can lead to deadly heat strokes if the body is not cooled quickly. “This can happen even with someone who is acclimated to a hot climate,” says Elizabeth Gardner, a sports medicine surgeon at the Yale School of Medicine.
Heat killed more than 2,000 Americans in 2023. That’s a 117 percent increase from the heat-related mortalities in the U.S. at the end of the 20th century, according to a study in the medical journal JAMA. And the problem is not just confined to the States: A heatwave this summer in France killed more than 5,000 people there. And British scientists have calculated that in the coming decades extreme heat may kill more than 30,000 citizens in England and Wales, significantly higher than heat deaths figures there today.
Hot days beyond an area’s norm also create problems for infrastructure constructed for a cooler planet. “Infrastructure is built for each specific region,” including the materials used for highways, railroads, and the like, Labe says. When steel railroad tracks heat and expand, trains must stop or slow significantly. Swing bridges can also be affected, as happened in Massachusetts last summer when 90-degree temperatures forced a bridge to remain down for several days over concerns that if the material expanded while open it would not be able to close.
Crops also wilt when many more 90-degree days occur in historically milder environments. “Thresholds for many crops are called ‘killing degree days,’” Neelin says. “The longer a heat wave, the worse off the crop is going to be.” And wildlife, which cannot escape to air-conditioned homes, is also stressed when the temperature spikes.
(Extreme heat triggers mass die-offs and stress for wildlife in the West.)
Even cool cities feel the heat
Cities are doing their best to prepare for the new reality of earlier, later, and more dangerous hot days. In recent years, Tucson has opened dozens of cooling centers for people who don’t have or can’t afford to run their air conditioning in the heat and has distributed cooling towels, water bottles, and other heat-relief items to unhoused residents. They’ve also begun to divert summer stormwater into basins where ground-cooling native trees and shrubs are planted, and to alter building codes to call for white roofs and other building changes that keep structures cooler.
Even Denver, a high-altitude city at the base of snow-capped mountains, has begun to plan for extreme heat. In 1970, Denver experienced just 32 days above 90 degrees, according to Climate Central. Since 2020, many years have seen at least 60 such days.
“Denver was built for a different climate than the one we have today,” says Elizabeth Babcock, executive director of the city’s office of climate action, sustainability, and resiliency, which was created in 2020—a year with a whopping 75 days over 90°F.
As part of its adaptation strategy, the city is distributing portable air conditioners (along with air purifiers to protect against the increased wildfire smoke) to low-income residents and is planting shade trees.
Denver is also working to mitigate future climate damage, Babcock says. It has created financial incentives for builders and homeowners to replace gas-powered furnaces with electric heat pumps and has set energy efficiency targets for skyscrapers. The city is also helping fund thousands of solar panels, heat pumps, EV chargers, and other clean energy solutions.
“We have changed the climate with our actions, so we have to adapt, but the future isn’t written yet,” Babcock says. “If we all lean into climate solutions, we can begin to turn the tide to prevent the worst.”
Still, even if every city—along with every government around the world—began taking significant actions to reduce the burning of fossil fuels, some additional warming is inevitable.
“In the middle of this century, cities that now have several weeks of more days over 90°F could see yet another several weeks,” Labe says. And the boundaries will creep northward, so areas not yet experiencing double-digit increases in the number of extreme days will likely soon, he says.
After that, how much our cities warm is up to us. “Further into the future, at the end of the century, that’s when the questions arise,” Labe says. “How much fossil fuel we continue to burn will determine how our summers look at the century’s end.”
